This disclosure is directed to systems and methods for charging particles with electron emission from carbon nanotubes and other nanotube variants, such as systems and methods for uniformly charging toner particles for use in document image forming, copying and printing devices, and/or pigment particles for use in electrostatic powder coat finishing applications.
Electrostatically charged particles are used as coloring agents in a number of different practical applications. These applications include electrophotographic and/or xerographic production and reproduction of documents throughout the electrophotographic industry, and electrostatic powder coat finishing processes employed in the painting and finishing industries. The advantages of using powder toners and/or coatings, when compared to liquid inks and other related coatings, include: ease of cleaning; reduction in the use of solvents; ability to collect overspray for potential reuse; and better control of uniformity of image or film thicknesses. In typical applications, electrostatically charged toner particles in image forming devices vary in size in a range of 3–15 μm in diameter, while electrostatically charged pigment particles used for powder coating have an average particle size in the range of approximately 30–40 μm in diameter. In either application, typically the particles are transported from a hopper to an apparatus in which an electrostatic charge is imparted to the particles prior to being further brought into contact with a charged or grounded surface upon which the particles are to be deposited and affixed.
Traditionally, triboelectric, or frictional electric, charging is typically the phenomenon and/or process which is used to impart the electrostatic charge to the particles. U.S. Patent Publication No. US 2004/0184840 A1 to Hays, published on Sep. 23, 2004 (hereinafter referred to as the “'840 Publication”), which is commonly assigned and the entire disclosure of which is incorporated herein by reference, describes the process of triboelectric charging and catalogs some of the advantages and disadvantages of triboelectric charging, specifically of toner particles.
The '840 Publication teaches that triboelectric charging is widely used in the eletrophotographic industry to charge toner particles. Triboelectric charging is performed by rubbing two dissimilar materials together, and in the course of such rubbing, a charge is transferred from one body to the other. A disadvantage is that the triboelectric charging phenomenon is very sensitive to surface conditions of the particle as well as temperature and humidity. Charge deposited on particles, particularly irregularly surfaced particles, can easily become non-uniform, resulting in localized areas of higher charge concentration on the particles. This increases the localized adhesion state of such particles to any surface, including the internal structural surfaces of the devices within which the electrostatically charged particles are manipulated. In electrophotographic and/or xerographic image forming devices, for example, this unwanted particle adhesion results in electrostatically charged particles such as, for example, toner particles, being “captured” along the pathways leading to an image on an image-carrying member, such as, for example, paper. This unwanted adhesion decreases the efficiency of the device because less than all of the available toner reaches the photoreceptor and subsequently the image-carrying medium. Reducing these adhesion forces results in delivering more toner to the image-carrying medium, thereby increasing the efficiency of transfer of the toner to the medium and the quality of the images produced. Additionally, there is a concomitant reduction in a requirement to clean adhered toner from the inside of the device and/or in a requirement to clean residual toner held by the photoreceptor.
For electrostatic powder coating applications, both tribocharging charging and ion charging guns are employed to charge the powder. For a tribocharging gun, an electrostatic charge is generated by friction between pigment particles and the gun barrel. When, for example, individual particles of epoxy powder used in powder coating move through a powder gun barrel, the particles rub against the inner surfaces and are electrostatically charged. For this reason, typical triboelectric powder guns used for powder coating have a long and tortuous barrel in order to increase the inside surface area, thereby increasing the triboelectric charging of the particles. A drawback of the tribocharging gun is that the types of powder materials that are compatible with the triboelectric charging process are limited.
For ion charging guns, a corona from a high voltage coronode wire is used to generate the electrostatic charge. Typical powder ion charging guns employed in powder coating applications require very high voltage power supplies of as much as 100 kV. Some of the ions are captured on the powder whereas the majority of ions are collected on the article to be coated. High ion charging of the coating is not desired since back ionization can occur that causes coating defects.
The '840 Publication proposes an ion-charging device for toner particles seeking to overcome disadvantages of conventional triboelectric charging and ion charging methods. The '840 Publication teaches a device for electrostatically charging toner particles by exposing the toner particles to unipolar gas ions emitted from opposing scorotrons that each consist of a coronode with pins of corona emitting points in combination with an electrically biased screen. The coronodes and screens are connected to power supplies through a network of high voltage diodes and resistors. A high voltage, e.g., in a range of 8 kV, AC power supply connected to the opposing scorotons through the diode and resistor network causes ions of a single polarity to be alternately generated by each scorotron during each half cycle in the region between the scorotrons where toner particles are entrained in an air stream. Thus, the toner particles accumulate charge of a single polarity as they flow through the zone between the scorotrons. The alternating electric field in the charging zone prevents deposition of toner on the coronodes and screens of the opposing scorotrons. The invention disclosed in the '840 Publication includes an apparatus for charging particles prior to being delivered to a development delivery device.
The '840 Publication explains that toner particle charging with ions has a number of advantages including insensitivity to toner material surface properties thereby making the proposed device and/or process adaptable to particles with irregular surfaces. The '840 Publication points out that the disclosed device more uniformly charges irregular or spherical-shaped toner particles with further advantages of a reduction in toner adhesion to surfaces. The toner charging device disclosed in the '840 Publication may be viewed as a separable component of the disclosed image forming devices and is, in fact, described as an “interface” between various methods for supplying toner to such a device and developing an electrostatic image with ion-charged toner. The '840 Publication describes that the disclosed charging devices and methods improve electrophotographic development, electrostatic transfer and the ability of the overall image-forming device to be cleaned.